Method for adjusting a resonator in an oscillator

ABSTRACT

A method for tuning a resonator in an oscillator is described. A dielectric serving as a resonator in the oscillator is trimmed in a targeted manner by laser pulses until a target frequency is reached. The lasers used are preferably excimer lasers or solid-state lasers.

BACKGROUND INFORMATION

[0001] The present invention is directed to a method for tuning aresonator in an oscillator according to the definition of the species ofthe independent claim.

[0002] It is known from U.S. Pat. No. 6,181,225 B1 to use a laser totrim a resonator (slab resonator), which was manufactured from metalusing a thick-film method, to tune the frequency of a resonator.

ADVANTAGES OF THE INVENTION

[0003] In contrast, the method according to the present invention fortuning a resonator in an oscillator having the features of theindependent claim has the advantage over the related art that the use ofa dielectric makes a higher quality of the oscillator possible, which isof particular value in the very high frequency range. It is thuspossible in particular to use the oscillator, of which the resonatoraccording to the present invention is a part, for higher frequencies inthe GHz range. The direct trimming of the dielectric, formed as aresonator pellet, results in improved reproducibility of the resonatorfrequency to be set. The method according to the present invention issuited in particular for the mass production of oscillators and it thusmakes a fast, safe, and simple method for frequency tuning of theresonators in oscillators possible.

[0004] The measures and refinements listed in the dependent claims makeadvantageous improvements of the method specified in the independentclaim for tuning a resonator in an oscillator possible.

[0005] It is advantageous in particular that the laser used for trimmingis operated as a pulsed laser in order to thus minimize the thermal loadon the oscillator circuit.

[0006] It is also an advantage that the oscillator frequency determinedby the dielectric is measured after a predetermined number of pulses inorder to thus adjust the predetermined oscillator frequency in aniterative process. In one refinement, it is possible to set theresonator frequency automatically using a control loop.

[0007] It is also an advantage that the oscillator according to thepresent invention has a metallic cover, which is necessary to stimulateoscillation of the oscillator since this metallic cover results inpositive feedback. The cover also has a bore through which the laser isable to aim at the dielectric in order to trim this dielectric. Thismakes direct trimming in the resonator possible, i.e., in the finishedcircuit of the oscillator, making it possible to immediately measure thesuccess of the trimming based on the oscillator frequency.

[0008] It is a further advantage that an excimer laser or a solid-statelaser, which may be laser diode-pumped, is used as a laser, such lasershaving the necessary performance density for the method according to thepresent invention and good trimming properties.

[0009] It is also an advantage that an oscillator is present, which istuned using the method according to the present invention, theoscillator having a metallic cover, a high-frequency transistor, forexample an HFET or a HBT, the electrical and electronic components beingconnected via microstrip lines and the dielectric being designed as acylindrical resonator pellet.

[0010] The laser used for the method according to the present inventionmust be capable of pulsed operation to minimize the thermal load on theoscillator as described above.

DRAWING

[0011] Exemplary embodiments of the present invention are depicted inthe drawing and explained in greater detail in the followingdescription.

[0012]FIG. 1 shows an oscillator system having a dielectric resonatorpellet;

[0013]FIG. 2 shows a resonator tuning using the method according to thepresent invention;

[0014]FIG. 3 shows an example of the tuning of the resonator frequencyin the form of a diagram;

[0015]FIG. 4 shows a flow chart of the method according to the presentinvention.

DESCRIPTION

[0016] For radar applications, in automotive engineering in particular,it is necessary to provide an oscillator that generates signals in thevery high frequency range, i.e., in the GHz range. Since methods such asDoppler frequency shift in particular are used to detect objects, aprecise determination and setting of the resonator frequency of theoscillator is necessary.

[0017] An oscillator has a passive and an active part. The active part,an amplifier, is a high-frequency transistor in this case, such as ahigh electron mobility transistor (HFET) or a hetero bipolar transistor(HBT), for example. These transistors are usually manufactured fromcompound semiconductors. The passive part is the resonator. It is formedin this case by a dielectric, whose equivalent electrical circuitdiagram may be formed from resistors, capacitors, and inductors, ifnecessary.

[0018] In manufacturing the oscillator, it is now possible to set theoscillator frequency, i.e., the frequency of the signal generated by theoscillator, by precisely setting the resonator. Since a dielectric isused as a resonator in this case, this dielectric must be changed by ageometric adaptation for setting the resonator frequency. According tothe present invention, this is achieved directly on the oscillatorcircuit by a laser used to trim the dielectric, the laser preferablybeing operated as a pulsed laser. Since the oscillator circuit is sealedby a metallic cover, this metallic cover has a bore through which thelaser may be aimed at the dielectric for trimming.

[0019]FIG. 1 shows an oscillator system having a resonator pellet. Theoscillator circuit, made up of a transistor T including its electrodesdrain D, source S and gate G, a resonator pellet DR and microstriplines, is situated on a substrate 3.

[0020] The transistor is connected to an output of the oscillator viamicrostrip lines 2 and also to dielectric resonator pellet DR. Resonatorpellet DR has a height D, which may be changed by trimming using alaser. The height, however, determines the electrical properties ofresonator pellet DR, i.e., its capacitance, inductance, and itsresistance, i.e., its impedance. The impedance in turn determines theoscillator frequency. Thus a change in height D brings about a change inthe oscillator or resonator frequency.

[0021] A high electron mobility transistor (HEMT), which is suited forgigahertz applications in particular, is used in this case as transistorT. As an alternative, it is possible to use a hetero bipolar transistor(HBT). Metallic cover 1 surrounding the oscillator circuit has a heightH and a bore (not shown) which lies directly above resonator pellet DR.The laser beam is guided through this bore to trim resonator pellet DR.A ceramic is used as the material for resonator pellet DR, in this casea combination of strontium, barium, and tantalum oxides. However, otherceramics, i.e., dielectrics, are also possible. After the tuning, cover1 may be sealed using an electrically conductive label.

[0022]FIG. 2 depicts how the resonator pellet is tuned. Resonator pellet4 is located directly beneath the bore, through which the laser beam isguided. Resonator pellet 4 is situated on a stripline 2, which islocated on a substrate 3. Cover 1 seals the oscillator circuit.

[0023] The substrate is made of a material suitable for millimeterwaves, e.g., Teflon-like materials or HF ceramics. The stripline ismanufactured by structuring a metallic layer, e.g., copper. The width ofsuch a stripline typically ranges from 0.5-1.0 mm. The thickness of thestripline is typically 40 μm. The diameter of the pellet is 2 mm, thethickness D is 1 mm.

[0024]FIG. 3 shows in a diagram that the resonator frequency, ingigahertz as a function of the number of laser pulses, shows measurementresults obtained with the method according to the present invention. Alargely linear relationship is seen in the resonator frequency being afunction of the number of laser pulses so that the thickness and thusthe oscillator frequency may be readily predicted based on the number oflaser pulses.

[0025] The wavelength of the laser must be adjusted to the absorptionspectrum of the ceramic (dielectric, i.e., the resonator pellet). Anexcimer laser, whose UV radiation is well-absorbed by the aforementionedceramic, is suited in particular for the ceramic referred to above. Thebeam profile must be adapted to the size of the pellet using masks andoptics.

[0026] The method according to the present invention is shown as a flowchart in FIG. 4. Initially in method step 5, resonator pellet 4 istrimmed for a predetermined period of time At, which corresponds to apredetermined number of laser pulses, 100 for example, using a laser, anexcimer laser, or a diode-pumped solid-state laser. NdYAG lasers, forexample, may be used as solid-state lasers. After material has beentrimmed from resonator pellet 4 for predetermined period of time δt, theresonator frequency is measured in method step 6. If the frequency iswithin a predetermined range for the target frequency, the tuning iscompleted in method step 8. This may be attained, for example, when theobtained resonator frequency deviates from the target frequency by 1%.The frequency of the oscillator is fed to a spectrum analyzer andmeasured either via a suitable measuring socket and a suitable adaptoror via the emission of a connected antenna.

[0027] If, however, it was determined in method step 7 that thisfrequency has not been reached, the trimming is then continued in methodstep 5 using the laser. This process then proceeds iteratively until thepredetermined frequency of the oscillator has been reached. This is asimple, fast, and reliable method for the production of suchoscillators.

What is claimed is:
 1. A method for tuning a resonator in an oscillator,a laser being used to tune the resonator, wherein a dielectric (DR, 4)of the resonator is trimmed using the laser until a predeterminedfrequency of the resonator is reached.
 2. The method as recited in claim1, wherein the laser used to trim the dielectric (DR) is operated as apulsed laser.
 3. The method as recited in claim 2, wherein, after apredetermined number of pulses, the frequency of the resonator ismeasured before the trimming is continued.
 4. The method as recited inone of the preceding claims, wherein the trimming by the laser isperformed through a bore in a metallic cover (1) of the resonator. 5.The method as recited in one of the preceding claims, wherein an excimerlaser or a solid-state laser is used as a laser.
 6. An oscillator, whichhas been tuned using the method as recited in one of claims 1 through 5,wherein the oscillator has a transistor (T), which is connected to thedielectric (DR) on a substrate (3) via microstrip lines (2), theoscillator being sealed by the metallic cover (1).
 7. The resonator asrecited in claim 6, wherein the dielectric (DR) is designed as acylindrical resonator pellet.
 8. A laser for implementing the method asrecited in one of claims 1 through 5, wherein the laser is operable as apulsed laser.